2/6 DEVELOPMENT OF ELASMOBRANCH FISHES. 



In Amphibians, the alimentary cavity is formed coincidently 

 with a true ingrowth of cells from the point where epiblast and 

 hypoblast become continuous, and from this ingrowth the dorsal 

 wall of the alimentary cavity is formed. The same ingrowth 

 causes the obliteration of the segmentation cavity. 



In the Elasmobranchs, owing to the larger bulk of the lower 

 layer cells caused by the food-yolk, these have been compelled 

 to arrange themselves in their final position during segmenta- 

 tion, and no room is left for a true invagination ; but instead 

 of this there is formed a simple split between the blastoderm 

 and the yolk. The homology of this with the primitive invagi- 

 nation is nevertheless proved by the survival of a number of 

 features belonging to the ancestral condition in which a true 

 invagination was present. Amongst the more important of 

 these are the following: (i) The continuity of epiblast and 

 hypoblast at the dorsal lip of the anus of Rusconi. (2) The 

 continuous conversion of indifferent lower layer cells into hypo- 

 blast, which gradually extends backwards towards the segmenta- 

 tion cavity, and exactly represents the course of the invagination 

 whereby in Amphibians the dorsal wall of the alimentary cavity 

 is formed. (3) The obliteration of the segmentation cavity 

 during the period when the pseudo-invagination is occurring. 



The asymmetry of the gastrula or pseudo-gastrula in Cyclo- 

 stomes, Amphibians, Elasmobranchs and, I believe, Osseous 

 Fishes, is to be explained by the form of the vertebrate bo'dy. 

 In Amphioxus, where the small amount of food-yolk present is 

 distributed uniformly, there is no reason why the invagination 

 and resulting gastrula should not be symmetrical. In other 

 vertebrates, where more food-yolk is present, the shape and 

 structure of the body render it necessary for the food-yolk to 

 be stored away on the ventral side of the alimentary canal. 

 This, combined with the unsymmetrical position of the anus, 

 which primitively corresponds in position with the blastopore 

 or anus of Rusconi, causes the asymmetry of the gastrula invagi- 

 nation, since it is not possible for the part of the ovum which 

 will become the ventral wall of the alimentary canal, and 

 which is loaded with food-yolk, to be invaginated in the same 

 fashion as the dorsal wall. From the asymmetry, so caused, 

 follow a large number of features in vertebrate development, 



